This project focuses on the mechanisms involved in cognitive control. By ~control~, we refer to the ability of the cognitive system to flexibly adapt to its behavior to the demands of particular tasks, favoring the processing of task relevant information over other sources of competing information, and mediating task relevant behavior over habitual, or otherwise prepotent responses. The work we propose is a direct extension of our previous efforts to understand and develop explicit models of this function of PFC, and its impairment in schizophrenia. Previously, we have hypothesized that PFC houses a mechanism for representing and maintaining context information. The primary goal of this project is to develop a more detailed theory of PFC function, and its interaction with other cortical systems involved in cognitive control. In particular, we will focus refinements on the mechanisms underlying active maintenance of representations within PFC, the nature of these representations, and the interaction between PFC and hippocampal systems. We will develop and evaluate gated attractor networks as a mechanism for active maintenance of context information. We will evaluate these systems for their ability to a)provide the capacity to maintain context representations in the face of intervening distractor items; b) be implemented using mechanisms that are consistent with data regarding the neuromodulatory functions of dopamine (DA); c)learn to identify and actively store task relevant representation using a reinforcement learning mechanism; d) account for the performance of both normal and schizophrenic subjects in delayed response tasks, and of frontal schizophrenic patients in the Wisconsin Card Sort task. We will also explore the nature of representations within PFC that support its role cognitive control. We will test the following hypotheses: a)Representations within PFC are categorical and combinatorial, and can be used to represent task relevant information at diverse levels of specificity. B)These properties derive directly from the requirement that they be actively maintained. C) The representational scheme in PFC complements that of the posterior neocortex, and can be used to effectively bias processing within the posterior system in accord with task demands. d) Representations with PFC can develop over training in response to task requirements. E) These characteristics can account for normal and impaired performance in neuropsychological tasks classical associated with frontal function, including verbal fluency and object sorting tasks. Finally we will extend our framework in order to account for the role of PFC in more complex cognitive tasks. Specifically, we will develop an integrated model of PFC and hippocampal function, that incorporates an existing model of the latter, and test whether this can account for: a)cognitive control in novel tasks b) the organizational influence of PFC representations over hippocampal storage and episodic memory c) the distinct contributions of PFC and hippocampus to cognitive impairments in neuropsychiatric disorders.